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Over slab polystyrene foam board insulation and clay tiles

hatcherfam | Posted in Green Building Techniques on

We are using polystyrene board insulation for an over slab application. Is 25 psi polystyrene foam board insulation adequate to support one layer 1/2 inch osb board, one layer of cement backer board topped with ceramic tile?

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  1. GBA Editor
    Martin Holladay | | #1

    Yes. More information here: Foam Under Footings.

  2. hatcherfam | | #2

    Thanks Martin. This article addresses under slab insulation. I'm wondering about over slab EPS boards and the integrity of clay or ceramic tiles on top of 25 psi EPS.

  3. Expert Member
    Dana Dorsett | | #3

    The rigidity of half-inch OSB on top of 25psi foam in full contact with a slab below is at least comparable to and probably BETTER than the flex of standard thickness OSB sub-flooring suspended on 16" o.c. joists. If the tile backer is rated for a standard subfloor, it's going to be good here, if...

    It's important to stagger the seams of the foam & OSB, as well as the seams of the backer-board & OSB, since those will be the flex points with the highest pressure & deflection. If you had the seams all alighned directly over one another you would have maximal shear forces at the weakest point in the foam layer, and it could develp rockering compression and cracking over time. If the seams of the OSB are a foot away from those of the foam, and the seams of the backer at least 6" away from those of the OSB it will have very little "give" to it.

  4. user-1140531 | | #4


    How do you plan to attach the foam, OSB, and cement backer board?

  5. albertrooks | | #5


    Here is a Cork alternative. This standard grade cork is R4/inch, takes a lot of load and has been used successfully as "over slab insulation" for years. It allows a lot of latitude of what you put on top of it.

    We've been developing a new informational website. it's not fully developed yet, but some technical specs are up: compression resistance, thicknesses, thermal conductivity, etc.

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